An aromatic polyimide film shows a high heat resistance as well as good electric characteristics, and is widely employed as material for electronic devices such as an electronic camera, a personal computer, and a liquid crystal display. For instance, the aromatic polyimide film is bonded to a copper foil via an adhesive such as an epoxy resin to produce a flexible printed circuit (FPC) or a substrate for tape automated bonding (TAB). Although the aromatic polyimide film is highly heat resistant, the conventional adhesive such as an epoxy resin has poor heat resistance. Accordingly, it is a problem that the produced copper foil/adhesive/polyimide film composite does not show satisfactory heat resistance.
In order to produce a copper foil/polyimide film composite having a high heat resistance, a number of improvements have been proposed. For instance, a copper metal is electrolytically plated on a polyimide film having no adhesive layer; a polyamide acid solution is coated on a copper foil, dried and heated to imidize the coated polyamide acid; or a thermoplastic polyimide is bonded to a copper foil by thermocompression.
U.S. Pat. No. 4,543,295 describes a polyimide composite (laminate) which is produced by bonding a polyimide film to a metal foil via a polyimide adhesive.
According to studies of the present inventors, however, the bonding strength of a polyimide composite produced by the above-mentioned process is not high.
Japanese Patent Provisional Publications H4-33847 and H4-33848 describe a method of bonding a multi-layered aromatic polyimide film to a metal foil by thermocompression. This process appears to be disadvantageous in that it requires a considerably high temperature and a high pressure for the thermocompression procedure. In particular, a temperature of higher than 300.degree. C. is required in a continuous process for firmly bonding a multi-layered polyimide film to a metal foil. Such a high temperature is disadvantageous because the continuous process generally employs a rubber roller which is not high in heat resistance.
Therefore, most of these known processes are not appropriately employable for continuously producing a metal foil/polyimide film composite in a long term operation.